The unusual ability of gecko lizards to climb on any vertical surface and hang from the ceiling with one toe has inspired scientific research for centuries. Recent studies revealed that the driving force for holding gecko lizards on a surface arises from the strong van der Waals forces (about 10 N·cm−2) induced by hundreds of thousands of aligned microscopic elastic hairs called setae (3-130 μm in length) containing hundreds of spatulae (0.2-0.5 μm in diameter) on a gecko's foot. The gecko has an unusually strong adhesion force upon pulling the setae parallel to the surface during attachment and a weak adhesion force perpendicular to the surface during detachment.
These findings have prompted researchers to fabricate microarrays of polymer pillars to mimic gecko feet. However, these polymeric dry adhesives (with a maximum achievable adhesive force of about 3 N·cm−2) are not comparable to gecko feet due to the inability to mimic the fine structure of geckos' setae and spatulae synthetically.
Work with macroscopic arrays of vertically-aligned multi-walled carbon nanotubes (VA-MWNTs) yielded a direction-independent adhesive force of 11.7 N·cm−2 for an as-synthesized VA-MWNT array, which is comparable to the overall adhesive strength observed for a gecko's foot. The VA-MWNT arrays were produced by chemical vapor deposition. (Fe was used as the catalyst, and the carbon source was a hydrocarbon gas (e.g., acetylene)). During the pyrolytic growth of the VA-MWNTs, the initially formed carbon nanotube (CNT) segments from the “base-growth” process grew in random directions and formed a “coiled/entangled” nanotube top layer on the resultant VA-MWNT array. The top layer of entangled nanotube segments prevented the underlying VA-MWNTs from contacting the target surface. It was demonstrated that the adhesion force could be increased by removal of the randomly-distributed CNT segments from the top surface of the VA-MWNT array through plasma etching. The necessity of the additional step makes this a more complicated process.
Efforts have been made recently to synthesize vertically-aligned single-walled carbon nanotube (VA-SWNT) arrays. The VA-SWNTs show potential for dry adhesion applications with additional electrical/thermal management capabilities because of their extremely high aspect ratio, exceptional mechanical strength, and excellent electronic and thermal properties. The smaller nanotube diameter could also allow VA-SWNT arrays to have more contact points per unit surface area compared to their multi-walled counterparts, leading to an enhanced adhesion force for the VA-SWNT dry adhesives.
However, the chemistry for synthesizing high quality VA-SWNT arrays consistently is still not mature. Although studies have reported that VA-SWNT arrays can be produced by pyrolysis of certain hydrocarbons in the presence of H2, H2O, or O2 activated by microwave, RF discharge, or hot filament, our attempts to reproduce VA-SWNTs by most of the reported methods were not successful. This was probably due to a narrow window of growth parameters associated with those reported methods.
Therefore, there is a need for an easy and reliable method for producing VA-MWNT and VA-SWNT arrays having a high shear adhesive force, and for dry adhesives using the VA-MWNT and VA-SWNT arrays.